This invention relates to improvement of an optical characteristics switching mechanism for switching optical characteristics of an optical system. This invention also relates to an optical head device and an optical disk reproduction/recording apparatus having the optical characteristics switching mechanism.
As is well known, an optical disk reproducing/recording system for reproducing/recording information from/in an optical disk as a recording medium has an optical head. The optical head records information in an optical disk by irradiating the optical disk with a laser beam and reads information stored in the optical disk by irradiating the optical disk with a laser beam and receiving the laser beam reflected from the optical disk.
In recent years, various types of optical disks have been developed. Among the new disks, a type of optical disk: a DVD (Digital Versatile Disk) remarkably attracts attention. A DVD has a diameter of 12 cm equal to that of the conventional CD (Compact Disk). A DVD, however, has a signal recording substrate as thin as 0.6 mm, unlike the conventional CD. In compensating the thinness, the signal recording substrate is doubled by adhering two signal recording substrates to each other on their back sides, to increase the thickness of the DVD to 1.2 mm, equal to the conventional CD.
With use of such thin signal recording substrates, the adverse effect due to the tilt of the optical disk can be reduced to remarkably improve the information recording density. More specifically, a DVD with two signal recording substrates adhered to each other can store a vast amount of information as large as 4.7 GB (Giga Bytes).
In the mean time, it is eagerly required for the optical system for reading information stored in a DVD to have compatibility with the system for the conventional CD so that the DVD optical system can read the information signal stored in the conventional CD. In order to attain the compatibility of the optical systems for the conventional CD and the DVD, the optical characteristics of the optical head in the DVD optical system need to be switched in reproducing DVD and in reproducing CD.
In order to switch the optical characteristics of the optical head, various methods as suggested below have been proposed: (1) a method by switching two objective lenses: a lens for a DVD, and a lens for a CD; (2) a method by forming a focus for reproducing a CD with use of an objective lens for a DVD by arranging hologram or the like; and (3) a method by arranging in a system an element for changing the numerical aperture of the objective lens for a DVD in order to reproduce/record information from/in a CD.
The first method, however, needs two objective lenses high in price, and thus is disadvantageous in an economical field of view. Further, it is very difficult to completely coincide the axial tilts of two objective lenses. In addition, the optical head body for containing the two objective lenses will inevitably increases in size, to fail the reduction of the optical head in size.
Similarly, according to each of the second and third methods, the optical system has an optical path in which the utility efficiency of the laser beam may be decreased. As a result, a part of the optical characteristics, which enables the reproduction of the DVD, may be deteriorated.
The switching means used for switching normal scope/panorama scope or used for zooming in a digital still camera can be also employed to switch the optical characteristics of the optical head. With use of such switching means, however, the body of the system will inevitably increase in size and it is thus very difficult to use such switching means used in the conventional digital still camera in such a compact device as an optical head in the DVD reproduction/recording system.